(1) Field of the Invention
The present invention relates to a heterojunction bipolar transistor and its manufacturing method.
(2) Description of the Related Art
A heterojunction bipolar transistor (hereinafter referred to as HBT) using a semiconductor of a large bandgap as an emitter has gained attention as a high-frequency analog device used for a cell phone and the like. Particularly, an InGaP/GaAs HBT using InGaP as an emitter has been commercially utilized as a device with high performance and high reliability because it has the following advantages: lower dependency on temperature of a current amplification factor (HFE) because of large valence band discontinuity (ΔEv); and less recombination current in the base because of no existence of the interface trapped density nor the DX center (See, for example, Japanese Laid-Open Patent Application Publication H5-36713 and Japanese Laid-Open Patent Application Publication H8-241896).
The device structure and its manufacturing method of a common InGaP/GaAs HBT will be explained below with reference to the drawings.
FIG. 1 is a cross-sectional view showing a structure of an InGaP/GaAs HBT.
As shown in FIG. 1, the InGaP/GaAs HBT has a multilayer structure including the following layers formed on a semi-insulating GaAs semiconductor substrate 800 in the following order: an n+-type GaAs sub-collector layer 810 formed by doping an n-type dopant with high concentration; an n-type GaAs collector layer 820 with low dopant concentration; a p-type GaAs base layer 830 with high dopant concentration; an n-type InGaP emitter layer 840; an n-type GaAs emitter cap layer 850; and an n-type InGaAs emitter contact layer 860 for forming an emitter electrode with low contact resistance.
Collector electrode 870, made of AuGe/Ni/Au or the like, for example, is formed on the sub-collector layer 810, a base electrode 871, made of a multilayer metal or the like including Pt, for example, is formed on the emitter layer 840 by diffusing thermally so as to come into contact with the base layer 830, and an emitter electrode 872, made of Ti/Pt/Au or the like, for example, is formed on the emitter contact layer 860.
Insulating film 880, made of SiO2, SiN or the like, is formed on the exposed surface, where the electrodes are not formed, of the sub-collector layer 810, the collector layer 820, the base layer 830, the emitter layer 840, the emitter cap layer 850 and the emitter contact layer 860.
FIG. 2A to FIG. 2D are cross-sectional views for explaining the manufacturing method of the HBT with the above structure.
First, as shown in FIG. 2A, the sub-collector layer 810, the collector layer 820, the base layer 830, the emitter layer 840, the emitter cap layer 850 and the emitter contact layer 860 are laminated on the semiconductor substrate 800 in this order.
Next, as shown in FIG. 2B, the emitter contact layer 860 and the emitter cap layer 850 are removed by wet etching using a mixed solution of phosphoric acid, hydrogen peroxide and water until the emitter layer 840 is exposed. As a result, an emitter island is formed.
Next, as shown in FIG. 2C, the emitter layer 840, the base layer 830 and the collector layer 820 are removed in this order by etching until the sub-collector layer 810 is exposed. As a result, a base island is formed.
Next, as shown in FIG. 2D, after the insulating film 880 is deposited over the surfaces of the emitter island, the base island and the sub-collector layer 810, contact windows that correspond to the emitter electrode, base electrode and collector electrode are opened. Then, the emitter electrode 872 is formed in the emitter contact window, the base electrode 871 is formed in the base contact window, and the collector electrode 870 is formed in the collector contact window. The InGaP/GaAs HBT is manufactured in the manner as mentioned above.
By the way, the applicability of the InGaP/GaAs-system HBT has been widened in recent years, and there is a demand for the HBT to have the characteristic of high power outputs. For example, as for a transmission amplifier of a cell phone particularly, the high power output of 3 to 4 W is especially required when the HBT is commercially utilized as a power device of a terminal transmitter of the GSM-system but not of the conventional CDMA-system.
However, there is a problem that a conventional InGaP/GaAs HBT breaks down more easily on higher power output.
Here, the breakdown of the HBT on operation for high power output will be explained below.
FIG. 5 is a diagram showing the Vc (collector voltage)−Ic (collector current) characteristics and breakdown voltage curve of the conventional InGaP/GaAs HBT. It should be noted that, in FIG. 5, dashed lines indicate the Vc−Ic characteristics of the conventional HBT at different base currents IB, while a dotted line indicates a breakdown voltage curve which is drawn by plotting the points at which the HBT is broken at respective base currents IB on high power output. In this diagram, the left side of the breakdown voltage curve is an area called a safe operation area (SOA).
FIG. 5 shows that the HBT is broken down on operation for high power output at the moment when the operation points of the HBT go beyond the SOA, namely, when the operation curves and the breakdown voltage curve intersect with each other. This breakdown is caused by sudden increase in Ic at a specific Vc, and this phenomenon in which Ic increases suddenly at the specific Vc is called avalanche multiplication.